This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and tend to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls. One particular problem arises due to the formation of pieces of solid metal known as "skulls" in the vicinity of the pool confining side plates or dams. These problems are exacerbated when efforts are made to reduce the superheat of the incoming molten metal. The rate of heat loss from the melt pool is greatest near the side dams due primarily to additional conductive heat transfer through the side dams to the roll ends. This high rate of local heat loss is reflected in the tendency to form "skulls" of solid metal in this region which can grow to a considerable size and fall between the rolls causing defects in the strip generally known as "snake eggs". Because the net rate of heat loss is higher near the side dams the rate of heat input to these regions must be increased if skulls are to be prevented. It has therefore been proposed to provide an increased flow of metal to these "triple point" regions (ie. where the side dams and casting rolls meet in the meniscus regions of the casting pool) by providing flow passages in the end of the core nozzle to direct separate flows of metal to the triple point regions. Examples of such proposals may be seen in U.S. Pat. Nos. 4,694,887, 5,221,511 and our earlier Australian Patent Application 35218/97 based on Provisional Application PO2367.
Although triple point pouring has been effective to reduce the formation of skulls in the triple point regions of the pool it has not been possible completely to eliminate the problem because the generation of defects is remarkably sensitive to even minor variations in the flow of metal into the triple point regions of the pool. We have now determined that significant flow changes are brought about by variation in the position in the ends of the core nozzles relative to the side dams which may be brought about by inaccurate location of the core nozzle during set up and by subsequent movement of the nozzle ends due to thermal expansion during casting. As the gap between the nozzle end and the side dam is reduced the downwardly inclined flow of metal from the triple point pouring passages in the ends of the nozzle impinges higher on the side dams. This can lead to the formation of skulls with subsequent snake egg defects or in extreme cases can cause the poured metal to surge upwardly in the reduced gap between the nozzle ends and side dams to spill over the upper edges of the side dams. The present invention enables this problem to be overcome by simple modifications to the manner in which the metal delivery nozzle is mounted and held in position.